1. Field of the Invention
The present invention relates to a red light fluorescent material and a manufacturing method thereof, and a white light luminescent device incorporating the red light fluorescent material. More particularly, the present invention relates to a red light fluorescent material with high color purity, high luminance, and high chemical stability and a manufacturing method thereof, and a white light luminescent device incorporating this red light fluorescent material.
2. Description of Related Art
With recent progress in green technologies, a white light emitting diode now has advantages of low power consumption, compactness, low driving voltage, mercury free, and so forth. Therefore, the white light emitting diode has been extensively applied in the backlight modules of flat panel displays and illumination. In order to enhance the light emitting performance of the white light emitting diode, research and development of fluorescent materials play a significant role. Different novel fluorescent materials have been provided as a result.
In a white light luminescent device disclosed in U.S. Pat. No. 5,998,925 by the Nichia company, the white light luminescent device mainly transforms blue light emitted by a blue light emitting diode into yellow light with use of the garnet fluorescent powder doped with cerium (Y3Al5O12:Ce3+, YAG:Ce) so as to generate white light by mixing the blue light and the yellow light. However, the white light generated by the blue light emitting diode and the garnet fluorescent material which is doped with cerium always has the problem of high color temperature. Especially, when an operating current is elevated, the problem of the increased color temperature is further worsened. Moreover, as a white light luminescent spectrum does not include a red-colored component, the white light generated by this method has poor color rendering property, and thus has insufficient color rendering property when used as the illuminating light source. For instance, a red object irradiated by this white light has a color of pale orange.
The above problem can be resolved by increasing the red-colored component in the white light luminescent spectrum. According to the international patent application WO 02/11173 A1 published by ORSAM, a light emitting diode with a light emitting wavelength of the UV light˜the blue light (370 nm˜480 nm) cooperates with a dual-fluorescent powder system constituted by red light fluorescent powder and yellow-green light fluorescent powder to generate the white light. Here, the chemical formula of the red light fluorescent powder is MS:Eu2+, where M represents Ba, Sr, Ca, Mg, or Zn. The chemical formula of the yellow-green light fluorescent powder is M*N*2S4:Eu2+, Ce3+, where M* represents Ba, Sr, Ca, Mg, or Zn while N* represents Al, Ga, In, Y, La, or Gd.
The white light generated by the aforementioned dual-fluorescent powder system helps resolve the problems of the increased color temperature and the insufficient color rendering property. However, the components of the fluorescent powder used herein all contain sulfide, which easily reacts with moisture in the air, so that the dual-fluorescent powder system has poor chemical stability. In addition, under long term irradiation of the UV light, the dual-fluorescent powder system degrades easily and has the problem of insufficient lifetime. Besides, as sulfide has poor thermal stability, the fluorescent powder that incorporates sulfide as the main component has limited applications.